1. Field of the Invention
This invention relates in general to vehicle braking systems, and in particular to a method of controlling a braking system having regenerative brake control.
2. Background of the Invention
Vehicular regenerative braking systems generally capture forward kinetic energy from braking events and use that recovered energy to accelerate the vehicle. By using energy recovered from braking, the energy efficiency of the vehicle is improved. Regenerative braking may be used on vehicles having hybrid or pure electric powertrain platforms. On many of these hybrid or pure electric powertrain vehicle platforms, maximum recovery of braking energy is required to make the vehicle viable in the market place. Ideally, these regenerative braking systems are generally transparent (unnoticed) to the driver.
Vehicles are commonly slowed and stopped with hydraulic brake systems employing friction wheel brakes. These systems vary in complexity but a base brake system typically includes a brake pedal, a tandem master cylinder, fluid conduits arranged in two similar but separate brake circuits, and wheel brakes in each circuit. The driver of the vehicle operates a brake pedal which is connected to the master cylinder. When the brake pedal is depressed, the master cylinder generates hydraulic forces in both brake circuits by pressurizing brake fluid. The pressurized fluid travels through the fluid conduit in both circuits to actuate brake cylinders at the wheels to slow the vehicle.
Base brake systems in conventionally fueled vehicles typically use a brake booster that acts during braking to provide additional force that assists the pedal force created by the driver. The booster can be vacuum or hydraulically operated. A typical hydraulic booster senses the movement of the brake pedal and generates pressurized fluid which is introduced into the master cylinder. The fluid from the booster assists the pedal force acting and increases the pressure of the fluid acting on the wheel brakes. Thus, the pressures generated by the master cylinder are increased. Hydraulic boosters are commonly located adjacent the master cylinder piston and use a boost valve to control the pressurized fluid applied to the booster. Typically the boost valve is connected with the booster in the master cylinder assembly and mechanically coupled to the brake pedal for proper operation.
Braking a vehicle in a controlled manner under adverse conditions requires precise application of the brakes by the driver. Under these conditions, a driver can easily apply excessive braking pressure thus causing one or more wheels to lock, resulting in excessive slippage between the wheel and road surface. Wheel lock-up leads to loss of directional control and possible greater stopping distances.
Advances in braking technology have led to the adoption of Anti-lock Braking Systems (ABS). In ABS, the system monitors wheel rotational behavior and selectively applies and relieves brake pressure in the corresponding wheel brakes in order to maintain the wheel speed within a selected slip range to achieve good braking force and maintain steering control by avoiding wheel lock-up. While such systems are typically adapted to control the braking of each braked wheel of the vehicle, some systems have been developed for controlling the braking of only a portion of the plurality of braked wheels.
Electronically controlled ABS valves, including apply valves and dump valves, are located between the master cylinder and the wheel brakes. The ABS valves regulate the pressure between the master cylinder and the wheel brakes during ABS braking. Typically, when activated, these ABS valves operate in three pressure control modes: pressure apply, pressure dump and pressure hold. The apply valves allow pressurized brake fluid into respective ones of the wheel brakes to increase pressure during the apply mode, and the dump valves relieve brake fluid from their associated wheel brakes during the dump mode. Wheel brake pressure is held constant during the hold mode by closing both the apply valves and the dump valves.
To achieve maximum braking forces while maintaining vehicle stability, it is desirable to achieve optimum slip levels at the wheels of both the front and rear axles. During vehicle deceleration different braking forces are generally required at the front and rear axles to reach the desired slip levels. Therefore, the brake pressures should be proportioned between the front and rear brakes to achieve the highest braking forces at each axle. In conventional braking systems of the past, this apportioning was accomplished by a proportioning valve, which typically proportioned front and rear brake pressure according to a fixed ratio. Braking systems may be provided with Dynamic Rear Proportioning (DRP) systems, which use the ABS valves to separately control the braking pressures on the front and rear wheels to dynamically achieve optimum braking performance at the front and rear axles under the then current conditions.
A further development in braking technology has led to the introduction of Traction Control (TC) systems. Typically, valves have been added to existing ABS systems to provide a brake system which controls wheel speed during acceleration. Excessive torque applied to wheels during vehicle acceleration leads to wheel slippage and a loss of traction. An electronic control system senses this wheel slippage and automatically applies braking pressure to the wheel cylinders of the slipping wheel to reduce the slippage and increase the traction available. In order to achieve optimal vehicle acceleration, pressurized brake fluid (e.g., from the ABS pump) is made available to the wheel cylinders even if the master cylinder is not actuated by the driver.
During vehicle motion such as cornering, dynamic forces are generated which can reduce vehicle stability. A Vehicle Stability Control (VSC) brake system improves the stability of the vehicle by counteracting these forces through selective brake actuation. These forces and other vehicle parameters are detected by sensors which signal an electronic control unit. The electronic control unit automatically operates pressure control devices (e.g., pump, boost valves, apply valves, and dump valves) to regulate the amount of hydraulic pressure applied to specific individual wheel brakes.
In electric vehicles, in order to extend vehicle range, it is typical to include some kind of regenerative braking system in the vehicle. A regenerative braking system seeks to recapture energy from a moving vehicle by converting kinetic energy to electrical energy and storing it in an energy storage device such as a battery. The vehicle is also slowed as a result of the process of recapturing energy. Most regenerative braking systems work by using an electromagnetic drive motor(s) as generators. The operation of one such system is coupled to a selector switch and the accelerator pedal. When the selector switch is set for regenerative operation, as the driver removes his foot from the accelerator pedal, the electric motor is de-energized and coupled to the battery charging circuit which places an electromagnetic load within the motor. This simultaneously acts to slow the vehicle as well as generate electricity that is returned to the batteries.
Regenerative braking systems are cooperatively controlled with the friction brakes to allow for maximum energy recovery during braking operations. As the driver applies the brake, brake torque is generated with frictional braking on the respective wheels and/or regenerative braking torque on a respective Driven axle. In order to maximize recovered energy, preference is given to the regenerative system. The regenerative braking system blends to the torque generated from the regenerative drivetrain with friction braking by controlling pressure in the foundation brake system to achieve a smooth deceleration.